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Praetors of Safety.

And the following table, deduced from practice, is fairly explained:

* Material.
	Dead Load.
	Live Load.
	Moving Load.

Wrought Iron and Mild Steel Hard Steel    ........
	3 i
	5 to 8 q to 8
	9 to 13
 I O tO I ^

Bronzes        ............
	0  ^ " 6 'tO Q
	10 to 15

Cast Iron and Brass   ......
	6 to 10
	10 to 15

Average Stresses adopted in practice.—We must now
sum up the results obtained in testing, as given by the best
authorities, and form a table of breaking and safe stresses.. But
as there are high and low qualities for each material, and samples
of each quality vary so much, our tabulations can only be the
averages of many averages.

Breaking Stresses.—Thus cast iron may vary from 5 to 15
tons per square inch in tension, 22 to 58 in compression, and 4
to 5 in shear. Wrought iron breaks at from 15 to 30 tons in
tension, and 10 to 22 tons in shear. The strength of steel increases
with the carbon it contains, but as a rule its elongation is
simultaneously decreased. Steel plates should have but J per
cent. Cementation steel reaches very high strengths, varying
from 40 to 67 tons per square inch in tension, some samples of
tool steel yielding 88 tons ; and tempering increases its strength.
Steel castings bear from 15 to 34 tons with reasonable elongation.
Capper depends on mechanical treatment. Cast, it supports 10
tons; rolled into plates, 14 tons ; and drawn into wire, 20 tons.
Brass has 8 to 13 tons per square inch tension, and gun metal
10 to 23 tons.

There is some difficulty in collecting good results for com-
pression. If the specimen be ductile it flattens out, and then,
as Rennie said, 'the resistance becomes enormous.1 Brittle
materials are more easily dealt with. Besides, tension has been
looked upon as a sufficient test for all materials, and thus the
compression and shear columns are in many cases vacant. In such
cases we may take compression = tension, and shear » 7 of tension.